The info about the watermark comes from my own personal experience. I own a typed, signed letter with the post marked envelope from Tolkien written on his Sandfield Road stationary to a little girl in 1964 (I was able to contact the original owner who stated they had lost the letter at Princeston University in the late sixties. I bought the letter from a seller who found the letter in a storage box at Princeton). The Basildon Bond watermark is clearly visible when held up to a light.

A good while ago I was interested in purchasing another letter from Tolkien offered on Ebay. The letter was advertised as also being on Tolkien's personal stationary from his Sandfield Road address and was dated 1955 if I remember correctly. I asked the seller to hold the letter up to a light and see if there was a watermark there. He replied that there was a Basildon Bond watermark present. I did not get the letter (it went much higher than I could pay for it).

Of course, this does not mean that Tolkien did not use stationary other than Basildon Bond during this period.

The info about the watermark comes from my own personal experience. I own a typed, signed letter with the post marked envelope from Tolkien written on his Sandfield Road stationary to a little girl in 1964 (I was able to contact the original owner who stated they had lost the letter at Princeston University in the late sixties. I bought the letter from a seller who found the letter in a storage box at Princeton). The Basildon Bond watermark is clearly visible when held up to a light.